This invention relates to apparatus for the preparation of a compound or an alloy having one component with a substantially higher vapor pressure than the other, preferably for the preparation of a semiconductor compound with elements selected from group IIIA and VB of the periodic table and more particularly gallium phosphide, in general and more particularly to an improved apparatus in which the reaction tube is stationary and the heating means of the system is movable.
Apparatus of the general type of the present invention is known in which the components or elements to be combined forming the compound are situated in a closed horizontal reaction tube with its end surrounded by hollow cylindrical heating ovens, one oven being provided on each end. The heating ovens are arranged in the axial direction of the tube, one behind the other at a predetermined spacing. At the portion of the tube situated between the two heating ovens, a separate heating device, generally a high-frequency heating coil, is provided. The entire system is disposed within a pressure vessel, also referred to as an autoclave, having an internal pressure which can be varied as a function of the pressure inside the tube.
As is well known, gallium phosphide has found much use recently in the manufacture of light-emitting diodes for the visible range because of its large band gap. Polycrystalline gallium phosphide, which is used as the starting material for the manufacture of suitable single crystals, is advantageously synthesized through the reaction of gallium with phosphorus in a closed system.
Typical of one manner of making polycrystalline gallium phosphide is the direct synthesis method using gallium and phosphorus at a temperature of about 1500.degree. C and a pressure of 6 to 35 bar as disclosed by Frosch and Derick in Journal of the Electrochemical Society, Vol. 108, page 251, 1961. The components of the semiconductor compound are placed in a quartz tube which is arranged in a furnace and is provided with a high frequency heating device. The high-frequency heating device is inductively coupled to a graphite boat located in the tube and containing one of the components. For the reaction, the boat with the semiconductor component, such as gallium, is moved through the inductively heated zone of elevated temperature within the heating device. After an additional pass, dense polycrystalline gallium phosphide containing, at the end of a synthesized bar, free gallium is obtained. The induction coil for the high-frequency heating device is brought into the pressure vessel radially and is therefore not movable in the axial direction of the system. Thus to obtain a zone melting of the gallium, the tube must be moved and is supported for this purpose in a separate guide tube.
At the high temperatures which are required for the synthesis of the semiconductor compound, the strength of the wall of the tube is considerably reduced. Because of this the tube is disposed in a pressure vessel having a pressure set higher than the pressure in the tube. Since the internal pressure of the tube cannot be measured, the setting of the necessary counterpressure in the autoclave over the wide range of possible operating pressures is difficult.
It has been discovered that with a design of this nature in which the tube is movable, undesired vibrations of the melt can occur leading to disturbances of the growth. As a result the prior art equipment of this nature cannot be used for the preparation of a single-crystal material. Furthermore, the zone of elevated temperature produced by the induction heating system wide. This makes impossible a zone-melting process having a continuously traveling melting zone used for purification or the like.
In view of these difficulties, it is the object of the present invention to provide an improved apparatus which is suitable for zone purification and for single-crystal semiconductor preparation.